Various kinds of flat panel displays that have lower weight and smaller volume than devices such as cathode ray tubes (CRT) have been developed in recent years. The flat panel displays include liquid crystal displays (LCD), field emission displays (FED), plasma display panels (PDP), organic light emitting displays, and the like. Among these, the organic light emitting display can emit light by recombination of electrons and holes. This display has advantages of fast response speed and low power consumption.
FIG. 1 is a circuit diagram of a pixel 10 of a conventional organic light emitting display. The pixel 10 of the conventional organic light emitting display emits light corresponding to a data signal supplied to a data line D when a scan signal is applied to a scan line S.
As shown in FIG. 2, the scan signals are sequentially applied to the scan lines S starting from a first scan line S1 to an nth scan line Sn. The data signals are applied to the data lines D synchronously with the scan signal. The pixel 10 receives the data signal and displays an image corresponding to the received data signal when the scan signal is applied.
Each pixel 10 includes an organic light emitting diode (LED), and a pixel circuit 12 coupled to the data line D, the scan line S and an anode electrode of the LED.
The anode electrode of the LED is coupled to the pixel circuit 12, and a cathode electrode of the LED is coupled to a second power supply source of voltage VSS. The LED generates light corresponding to an electric current supplied from the pixel circuit 12.
The pixel circuit 12 includes a first transistor M1 coupled to the data line D and the scan line S, a second transistor M2 coupled between a first power supply source of voltage VDD and the LED, and a storage capacitor C coupled between a gate and a source of the second transistor M2.
A gate terminal of the first transistor M1 is coupled to the scan line S, and a source terminal thereof is coupled to the data line D. A drain terminal of the first transistor M1 is coupled to the storage capacitor C. The first transistor M1 is turned on to supply the data signal supplied from the data line D to the storage capacitor C when the scan signal is applied to the scan line S. As a result, a voltage corresponding to the data signal is charged in the storage capacitor C.
A gate terminal of the second transistor M2 is coupled to the storage capacitor C, and a source terminal thereof is coupled to the first power supply source of voltage VDD. A drain terminal of the second transistor M2 is coupled to the anode electrode of the LED. The second transistor M2 controls amount of electric current which flows from the first power supply source of voltage VDD to the LED corresponding to a voltage value stored in the storage capacitor C. The LED generates light of luminance corresponding to amount of the electric current supplied from the second transistor M2.
However, the conventional pixel circuit 12 has a disadvantage in that images of uniform luminance cannot be displayed due to a difference between threshold voltages of the second transistors M2 used in different pixels 10. A plurality of pixels 10 are arranged in an image display portion which displays images. In order for the image display portion to display images of uniform luminance, the threshold voltages of the second transistors M2 in the plurality of pixels 10 should be identical to one another. However, due to processing errors during fabrication, the threshold voltages of the second transistors M2 vary. As a result, the image display portion cannot display images of uniform luminance.